A Thrilling Adventure Unfolds as the World Marches Off to War : SONS OF THE MORNING, <i> by DeWitt S. Coop,</i> W.W. Norton, $19.95; 352 pages

Imagine a William Overgard novel without the wackiness; that is, imagine a thrilling adventure with plenty of one- and two-engine propeller airplanes and all of World War II as a diorama backdrop, with mysterious lakes in the north Canadian wilderness, mad German scientists, ancient worldly courtesans and plucky young damsels who can fly those vintage crates as well as the best bush pilots. Throw in atomic energy, the devil himself, a hidden crypt, and you’ve got DeWitt Coop’s “Sons of the Morning,” perfect for a rainy winter afternoon or a day at home with the flu.

At first we meet--in 1939, just before the outbreak of the war--the dour and creepy Rene Montour, a bush pilot up there in Canada who flies with a permanent chip on his wing--so to speak--because he’s Canuck and the Anglo-Canadians treat him like a soggy croque monsieur . Montour is out in this strange wilderness to keep track of a mysterious American expedition on the banks of remote Lake Tissinabi.

Montour feels strongly that these Americans aren’t what they say they are. So when he flies in under a lowering, threatening sky and finds their camp deserted, he’s not surprised. He does discover the diary of the head archeologist, a man named Dowd. Although Montour can’t read English, he craftily manages to find out that one of the words in the diary is gold.

A cryptic, circular drawing decorated with another language seems to confirm that yes, there is gold in the ominous cliff that juts above the north bank of the lake. Montour, in his savage, unevolved way, is overjoyed. He nabs the notebook and plans to find the gold, set up his own flying agency and never take orders again.

No worries: I’m not giving away the plot. You couldn’t give it away; it sticks to your fingers like saltwater taffy. Flash forward to Britain during the war. A reluctant and unpatriotic Montour is flying behind enemy lines to rescue British spies. His plane crashes. Before he dies, he hands the Dowd notebook to a man who calls himself Jason, because he’s prey to lofty thoughts: When not distracted by war, he ponders the meaning of life and searches--metaphorically, of course--for the Golden Fleece.

But now, Jason is stuck in the war, behind those enemy lines, badly wounded. It’s pretty darn lucky that his childhood home is only a few kilometers away. Jason--half-French, half-English--recovers in his old family barn. In a direct knockoff of “Flying Colours,” Volume III of C. S. Forrester’s Horatio Hornblower series, Jason frolics with a French beauty while the rest of the family (except his odious uncle) covers for him and saves him from harm.

When Jason reads the Dowd notebook, he can only think of the good they might do, and where they come from. For some reason, the diagram reminds him of something he’s seen in the Dordogne, near the cave paintings of Lascaux.

But then the notebook--ever more battered--falls into the hands of a mad Nazi scientist obsessed with inventing an atomic bomb before the Americans do. He, in turn, looks upon the diary and the accompanying diagram as the secret of nuclear fission and almost destroys his already unsteady brain in a desperate effort to “decode” its pages.

So this novel turns out to be a pleasing hybrid, a cross between “The Treasure of the Sierra Madre” and “Close Encounters of the Third Kind.” Something beyond the purely natural is going on at the top of that cliff by the remote Canadian lake, and about half a dozen people are inexorably drawn to it.

Dowd, we discover, was bored with the same old civilizations, and was trying to cut back to the very beginning of Judeo-Christian mankind, to decipher the secrets of Genesis. But the “Sierra Madre” scenario plays out as well. There’s no point in going on a quest if you’re a coarse churl like that Canuck bush pilot, or a nut case like the Nazi genius who wants to destroy the world.

This is a hale and hearty kind of book. The men are handsome and funny, the women voluptuous and intelligent. Good and evil are easily defined. But William Overgard, up there in adventure heaven, must be tapping his foot impatiently, knowing he could have done this so much better.

Next: Constance Casey reviews “Native American Testimony,” edited by Peter Nabokov (Viking) .